This invention relates to the operation of power plants whose primary source of power is a liquid metal fast reactor and to reactors for such plants. This invention has particular relationship to the fueling of the reactors for such plants. While this invention is uniquely applicable to plants whose reactors are of the liquid metal fast type, it is realized that in its broader aspects, it may find application to plants whose reactors are of other types such as pressurized water reactors or boiling water reactors or gas-cooled reactors. To the extent that this invention is applied to, or find use in, plants having reactors of other types than liquid metal fast reactors or to the reactors of such other plants, such application and use is within the scope of equivalents of this invention.
A liquid metal fast nuclear reactor (LMFR) includes a core of fissile fuel and fertile material. The fissile fuel is usually plutonium oxide. The plutonium in the PuO.sub.2 includes a number of isotopes, Pu238, Pu239, Pu240, Pu241, Pu242. The Pu239 and Pu241 are fissionable. The Pu239 constitutes about 86% of the Pu and the Pu241 about 2%. The fissile fuel is in fuel rods, each rod including cladding of stainless steel or zirconium alloy which contains pellets of mixed plutonium and uranium oxides referred to as [(Pu-U)O.sub.2 ] and pellets of UO.sub.2. In both cases, the UO.sub.2 contains depleted uranium. The UO.sub.2 serves as fertile material; the content of U235 in this uranium is about 0.2%. The content of PuO.sub.2 in the fuel rods is about 12 to 25%; the remainder is UO.sub.2. The core also includes blankets of depleted uranium as fertile material circumferentially and on the top and bottom. Alternatively, the fissionable fuel may be UO.sub.2 enriched between 12 and 25% in U235 mixed with UO.sub.2 with depleted uranium. Plutonium and uranium carbide may be used in place of the oxides.
Typically, the core may be regarded as having the shape generally of a circular cylinder, although strictly in transverse section it is polygonal with a large number of sides. The fissile fuel rods are assembled in a generally cylindrical unit. The blankets may include a ring encircling the fissile cylinder externally and discs disposed on the bases of the fissile cylinder. The fissile fuel rods may also be mounted in a ring including a blanket section in the center in addition to the external blanket sections. The coolant for the LMFR is a liquid metal, typically liquid sodium.
Liquid metal fast reactor cores have a high linearpower density capability. Linear-power density is defined as the number of kilowatts per foot (kw/ft) of fuel rod. In accordance with the teachings of the prior art, LMFRs were designed to take advantage of their high power density capability. Core volumes were kept to a minimum to obtain an average linear-power density of about 7.5 kw/ft for oxide fuel and 15 kw/ft for carbide fuel. This minimization of core volume has resulted in fluence and burnup levels which limit the fuel assembly lifetimes to the range of three to five years. In addition, the refueling interval has usually been limited, by the available control system worth, to about one year because of the high reactivity loss with burnup.
The demand for frequent refueling has serious disadvantages. The refueling is itself a complex process involving removal of the head of a reactor and replacement of radioactive fuel rods in a radioactive environment. It is also necessary to handle the highly reactive coolant which rapidly turns into caustic alkali in air. The labor cost is substantial and the reactor is shut down for relatively long intervals. Frequent refueling also incurs front-end fuel losses since partially burned fuel assemblies are removed during the first few refueling cycles. In an annular refueling cycle in which one fourth of the core is replaced each year, the first, second and third refuelings remove fuel that is only one-quarter, one-half, and three-quarters burned respectively. In this case, the loss is three-eighths of a core; in case of refueling over five years, this loss is two-fifths of a core. The loss may be recovered by reprocessing, but reprocessing is not available in the United States, and if it becomes available, it would be costly. The construction of a prior art power plant with an LMFR is complicated and the cost is materially increased by the necessity of providing the fuel handling machinery, the buildings to contain this equipment and the facilities for moving this equipment back and forth between the building and the reactor. In addition, there must be fuel storage pools and cooling equipment to handle the annually removed batches of fuel which are still highly fissionable and are also highly radioactive. Another disadvantage arises from the deterioration of the cladding of the fuel rods and of the fuel by crumbling which results from operation at the high linear power density.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a method of operating a power plant whose primary power source is an LMFR in such a way that refueling shall not be required over the life of the LMFR. It is also an object of this invention to provide an LMFR for practicing this method. An ancillary object of this invention is to provide a method of operating a power plant whose primary source is an LMFR in such a way that infrequent refueling over the life of the plant, typically at intervals of at least ten years, shall be required.